The copolymerization of vinyl monomers, particularly styrene and acrylonitrile is well known, as are the processes for their production. However, there are still many inherent problems which have either been unsolved, or in many cases, solved unsatisfactorily.
In a copolymerization reaction, precise control is desirable, if not essential, to maintain the quality of the product. For example, if monomer X is to be copolymerized with monomer Y, a wide range of copolymers are possible. The X monomer could comprise from 1 to 99% of the copolymer. Incident to this is a wide variation in the physical and chemical properties of the product copolymers. Accordingly, precise control is required such that the final product consists essentially of copolymers having a substantially uniform X:Y ratio. The closer the uniformity, or the narrower the range of variance, the better the product. Even in those instances where a wide range is tolerated, it is well known that a narrower range of variance results in a better product.
In a typical continuous mass copolymerization process, the monomers feed is introduced into a reaction vessel. The copolymerization is achieved under elevated temperature and pressure conditions. The reaction vessel is the first major area where control is essential. A uniform temperature needs to be maintained to produce a uniform product. The resultant fluid contains the produced copolymer and part of each of the monomers. Withdrawal of the fluids from the reaction vessel presents the second problem area. Since the copolymerization reaction in the reaction vessels is not complete, some monomers are still present in the withdrawn fluids. If uncontrolled, some copolymerization would continue to take place to produce a copolymer having a different monomers ratio due to the different temperatures and pressure conditions. Thus, control of this continued copolymerization is essential to keep it to a minimum. Further, if any continued copolymerization occurs, control over the conditions should be exercised such that the produced second stage copolymer has substantially the same monomers ratio.
After the withdrawn fluids are cooled, the separation of the copolymers from the monomers and other diluents takes place. This is normally achieved by applying heat to vaporize the undesired constituents. Thus, a third problem area is present. Heat application would cause copolymerization or further reaction of the constituents which leads to the contamination of the final product. Thus, this third polymerization should be kept to a minimum.
For example, in the case of SAN polymerization, it is known that precise control is essential to the production of a product having acceptable properties. If the acrylonitrile content of copolymer in a single product varies by more than about 4%, the different copolymers become incompatible with one another, resulting in an unacceptable, hazy product. Because styrene and acrylonitrile monomers polymerize at different rates, careful control is needed not only during the polymerization stage, but also during the subsequent purification stages. Especially in these latter processing stages, it is essential to maintain uniformity of the product, because even small amounts of copolymer product having a high acrylonitrile content can cause a yellow discoloration of the entire product, due to cyclization of adjacent pendant acrylonitrile groups upon heating of the copolymer, e.g., even during subsequent thermoforming steps.
This is an important factor in the post-polymerization treatment of the product in a SAN polymerization process, e.g., the removal of residual monomer from the polymer, known as devolatilizing the polymer. Once the polymer/monomer mixture leaves the reactor there is great risk of producing high-acrylonitrile-containing polymer, due to the uneven rates of polymerization for the two different monomer species and the high temperatures which are utilized for devolatilization. In conventional processes, devolatilization is typically carried out with thin film devolatilizing equipment, such as the so-called "Film Truder", which evaporates the liquid monomer very rapidly to minimize further polymerization. This thin film equipment, however, is relatively expensive and requires an inordinate amount of maintenance, e.g., at least once daily, because of its many moving parts and the extensive seals characteristic thereof. This maintenance requires, therefore, that the polymerization line be shut down or that some measure be taken to hold material upstream during maintenance. The result is an uneven quality of product and added expense for equipment and operation.